The invention relates to a double belt press with a hydrostatic belt support, in which the pressing belts are supported by support strips.
Continuous double belt presses are known from, for example, EP-A-544645. They are used in particular for the manufacture of webs or panels made of materials which are completely or partially free-flowing on passage through the double belt press. In particular plastics, such as e.g. thermoplastics or duromers passing through thermoplastic phases, which may possibly contain fillers or reinforcing fibers, are materials which may be used. It is also possible to have a reinforcing web running into the double belt press, onto which the materials are applied before running into the double belt press, for example, in the manufacture of thermoplastics reinforced with glass-fiber mats.
In general, the materials are supplied to the entrance point of the double belt press without pressure. A pressure is exerted on the material web only in the gap between the pressing belts. In isobaric presses, the pressure is exerted by a fluid pressure medium, such as e.g. hydraulic liquids or compressed air, in pressure chambers on the side of the pressing belts facing away from the material. In this case, pressure chambers are delimited by one side of the pressure plates, the respective pressing belt and annular seals between the pressure plate and the pressing belt. Consequently, the hydrostatic pressure of the pressure medium acts directly on the respective pressing belt which transmits this pressure as a diaphragm directly to the material web in the pressing gap, which web is in turn supported on the opposite pressing belt. The mutually opposite pressure chambers of both pressing belts must therefore be acting with the same pressure. Consequently, the material web begins the passage through the press without pressure, then passes into pressing zones of the respectively set pressures, and then leaves the press without pressure again.
If the material web in the double belt press is in a free-flowing state, it tends to flow according to the pressure gradient. This means that for the part of the press which, in the belt running direction, lies in front of the zone of a greater pressure, the free-flowing material would tend to flow back counter to the belt running direction. This can be counteracted only by viscosity forces. In the region of the pressure build-up there is a relative speed between the material and the pressing belts which is counter to the transport direction. Consequently, the pressure in the material is built up ensuring that a pressure already prevails before the respective pressure zone is reached. This is contradictory to the principle of the isobaric press, in which identical pressure is intended to prevail everywhere in a pressure zone. As a result of the premature pressure build-up, the pressing belts are forced against pressure chamber walls, seals or heat transmission bridges. This is particularly disadvantageous because the machine parts are not designed for sliding under high pressure and thus are subject to excessive wear.
To solve the above problem, it is proposed in EP-A-544,645 to support the pressing belts with a roller bed. An areal force prevails on the material side but, on the support side, only a linear pressure is transmitted via the pressure lines between belt and support rollers. The Hertzian stress resulting from the contact lines limits the application of this principle. Both the belt surfaces and the support rollers are constantly varyingly stressed close to the limits of elasticity by the Hertzian stress in the contact lines, which immediately leads to fatigue phenomena particularly in the pressing belts. Moreover, the pressing belts are subject to great bending stresses as they run over the support rollers, which leads to increased wear of the pressing belts.